1. Field of the Invention
The present invention relates to a single-focus lens, for example, used as an imaging lens in an electronic camera (digital camera).
2. Description of the Related Art
In recent years, an electronic camera which images a subject using a solid-state image sensor such as a charge-coupled device (CCD) in place of a silver-salt film has become popular. The electronic camera falls into two types: still camera and video camera. The still camera images still pictures, whereas the video camera images motion pictures. Imaging lenses described in Japanese Unexamined Patent Application Publication No. Hei 8-152555 and No. 9-90213 are examples of conventional imaging lenses used in such electronic cameras. Described in Japanese Unexamined Patent Application Publication No. Hei 8-152555 is an invention regarding an imaging lens having six lenses made of glass. Further, Japanese Unexamined Patent Application Publication No. Hei 9-90213 describes an invention regarding an imaging lens which comprises five lenses: four lenses made of glass and one lens with an aspheric surface made of plastic.
Miniaturization of the electronic camera is in demand as is the camera using silver-salt films. Therefore, desirably the imaging lens used in the electronic camera is simply constituted and downsized having a shorter overall length. Further, desirably the imaging lens is manufactured at a low cost. Conventional imaging lenses, however, do not fully satisfy these requirements. For example, the imaging lens described in Japanese Unexamined Patent Application Publication No. Hei 8-152555 has six lenses made of glass. Six lenses are rather too many and the use of glass for all lenses results in high manufacturing costs. The imaging lens described in Japanese Unexamined Patent Application Publication No. Hei 9-90213 has five lenses and one of these lenses is made of plastic with the aim of cost reduction. However, its overall length is long, thus fails in downsizing.
It is considered that when the imaging lens is constituted having a plurality of lenses with aspheric surfaces, the number of lenses is reduced and its overall length is shortened. The lenses with aspheric surfaces are desirably made of plastic rather than glass in view of costs and productivity. With the lens made of plastic, a change in optical performance due to environmental changes such as changes in temperature or humidity is large as compared to that with the lens made of glass, so that its power has to be set small in order to reduce the changes in optical performance. Thus, when a plurality of aspheric surfaces is used in the imaging lens, desirably, the lenses are made of plastic and power is distributed appropriately considering environmental changes. However, the imaging lens satisfying these requirements has not been developed heretofore.
The present invention has been achieved in view of the above problems and its purpose is to provide a single-focus lens in which cost reduction and a simple configuration with a shorter overall length are realized as well as preferable optical performance is achieved.
In the single-focus lens of the invention, consecutively arranged from the object side are a first lens element with a meniscus shape having a concave which faces the object side, a second lens element with a meniscus shape having at least one aspheric surface, a third lens element, with two concaves, a fourth lens element, with two convexes and a fifth lens element with a meniscus shape having a concave which faces an image side, the fifth lens element having at least one aspheric surface.
In the single-focus lens of the invention, the second lens element and the fifth lens element are preferably made of optical plastics in view of productivity of aspheric surfaces.
In the single-focus lens of the invention, for the sake of reducing deterioration in optical performance due to environmental changes, desirably the second lens element and the fifth lens element, both of which are made of optical plastics, satisfy conditional expressions xe2x88x920.02 less than 1/f2 less than 0.02 and xe2x88x920.02 less than 1/f5 less than 0.02 when f2 represents a focal length of the second lens element and f5 represents a focal length of the fifth lens element.
In the single-focus lens of the invention, in order to correct chromatic aberration, it is desirable to satisfy conditional expressions 1.70 less than nd1, 35 less than vd1, 35 less than vd3, and 35 less than vd4 when the ndi represents a refractive index of the first lens element for a wavelength of d-lines, the vd1 represents Abbe number of the first lens element for the wavelength of d-lines, the vd3 represents Abbe number of the third lens element for the wavelength of d-lines, and the vd4 represents Abbe number of the fourth lens element for the wavelength of d-lines.
In the single-focus lens of the invention, the aforementioned configuration realizes a single focus-lens which has a less number of lenses and a shorter overall length and in which various kinds of aberrations are preferably corrected.
Other and further objects, features and advantages of the invention will appear more fully from the following description.